248 research outputs found
The very flat radio - millimetre spectrum of Cygnus X-1
We present almost-simultaneous detections of Cygnus X-1 in the radio and mm
regimes, obtained during the low/hard X-ray state. The source displays a flat
spectrum between 2 and 220 GHz, with a spectral index flatter than 0.15
(3sigma). There is no evidence for either a low- or high-frequency cut-off, but
in the mid-infrared (~30 microns) thermal emission from the OB-type companion
star becomes dominant. The integrated luminosity of this flat-spectrum emission
in quiescence is > 2 x 10^{31} erg/s (2 x 10^{24} W). Assuming the emission
originates in a jet for which non-radiative (e.g. adiabatic expansion) losses
dominate, this is a very conservative lower limit on the power required to
maintain the jet. A comparison with Cyg X-3 and GRS 1915+105, the other X-ray
binaries for which a flat spectrum at shorter than cm wavelengths has been
observed, shows that the jet in Cyg X-1 is significantly less luminous and less
variable, and is probably our best example to date of a continuous, steady,
outflow from an X-ray binary. The emissive mechanism reponsible for such a flat
spectral component remains uncertain. Specifically, we note that the radio-mm
spectra observed from these X-ray binaries are much flatter than those of the
`flat-spectrum' AGN, and that existing models of synchrotron emission from
partially self-absorbed radio cores, which predict a high-frequency cut-off in
the mm regime, are not directly applicable.Comment: Accepted for publication in MNRA
Programmability in AIPS++
AIPS++ is an Astronomical Information Processing System being designed and implemented by an international consortium of NRAO and six other radio astronomy institutions in Australia, India, the Netherlands, the United Kingdom, Canada, and the USA. AIPS++ is intended to replace the functionality of AIPS, to be more easily programmable, and will be implemented in C++ using object-oriented techniques. Programmability in AIPS++ is planned at three levels. The first level will be that of a command-line interpreter with characteristics similar to IDL and PV-Wave, but with an intensive set of operations appropriate to telescope data handling, image formation, and image processing. The third level will be in C++ with extensive use of class libraries for both basic operations and advanced applications. The third level will allow input and output of data between external FORTRAN programs and AIPS++ telescope and image databases. In addition to summarizing the above programmability characteristics, this talk will given an overview of the classes currently being designed for telescope data calibration and editing, image formation, and the 'toolkit' of mathematical 'objects' that will perform most of the processing in AIPS++
A highly polarised radio jet during the 1998 outburst of the black hole transient XTE J1748-288
XTE J1748-288 is a black hole X-ray transient which went into outburst in
1998 June. The X-ray lightcurves showed canonical morphologies, with minor
variations on the ``Fast Rise Exponential Decay'' profile. The radio source,
however, reached an unusually high flux density of over 600 mJy. This high
radio flux was accompanied by an exceptional (>20%) fractional linear
polarisation, the variability of which was anti-correlated with the flux
density. We use this variability to discuss possible depolarisation mechanisms
and to predict the underlying behaviour of the (unresolved) core/jet
components.Comment: Accepted for publication in MNRA
V723 Cas (Nova Cassiopeiae 1995): MERLIN observations from 1996 to 2001
MERLIN observations of the unusually slow nova V723 Cas are presented. Nine
epochs of 6-cm data between 1996 and 2001 are mapped, showing the initial
expansion and brightening of the radio remnant, the development of structure
and the final decline. A radio light curve is presented and fitted by the
standard Hubble flow model for radio emission from novae in order to determine
the values of various physical parameters for the shell. The model is
consistent with the overall development of the radio emission. Assuming a
distance of 2.39 (+/-0.38) kpc and a shell temperature of 17000 K, the model
yields values for expansion velocity of 414 +/- 0.1 km s^-1 and shell mass of
1.13 +/- 0.04 * 10^-4 Msolar. These values are consistent with those derived
from other observations although the ejected masses are rather higher than
theoretical predictions. The structure of the shell is resolved by MERLIN and
shows that the assumption of spherical symmetry in the standard model is
unlikely to be correct.Comment: 6 pages, 6 figure
ASCA Observations of the Jet Source XTE J1748-288
XTE J1748-288 is a new X-ray transient with a one-sided radio jet. It was
observed with ASCA on 1998/09/06 and 1998/09/26, 100 days after the onset of
the radio-X-ray outburst. The spectra were fitted with an attenuated power-law
model, and the 2-6-keV flux was 4.6 * 10^{-11} erg s^{-1} cm^{-2} and 2.2 *
10^{-12} on 09/06 and 09/26, respectively. The light curve showed that the
steady exponential decay with an e-folding time of 14 days lasted over 100 days
and 4 orders of magnitude from the peak of the outburst. The celestial region
including the source had been observed with ASCA on 1993/10/01 and 1994/09/22,
years before the discovery. In those period, the flux was < 10^{-13} erg s^{-1}
cm^{-2}, below ASCA's detection limit. The jet blob colliding to the
environmental matter was supposedly not the X-ray source, although the emission
mechanism has not been determined. A possible detection of a K line from highly
ionized iron is discussed.Comment: 11 pages, 4 figures, submitted to ApJL. Fig2 is replaced with correct
on
Spectral evidence for a powerful compact jet from XTE J1118+480
We present observations of the X-ray transient XTE J1118+480 during its
Low/Hard X-ray state outburst in 2000, at radio and sub-millimetre wavelengths
with the VLA, Ryle Telescope, MERLIN and JCMT. The high-resolution MERLIN
observations reveal all the radio emission (at 5 GHz) to come from a compact
core with physical dimensions smaller than 65*d(kpc) AU. The combined radio
data reveal a persistent and inverted radio spectrum, with spectral index
\~+0.5. The source is also detected at 350 GHz, on an extrapolation of the
radio spectrum. Flat or inverted radio spectra are now known to be typical of
the Low/Hard X-ray state, and are believed to arise in synchrotron emission
from a partially self-absorbed jet. Comparison of the radio and sub-millimetre
data with reported near-infrared observations suggest that the synchrotron
emission from the jet extends to the near-infrared, or possibly even optical
regimes. In this case the ratio of jet power to total X-ray luminosity is
likely to be P_J/L_X >> 0.01, depending on the radiative efficiency and
relativistic Doppler factor of the jet. Based on these arguments we conclude
that during the period of our observations XTE J1118+480 was producing a
powerful outflow which extracted a large fraction of the total accretion power.Comment: Accepted for publication as a Letter in MNRA
V4641Sgr - Super-Eddington source enshrouded by an extended envelope
An optical spectroscopy of an unusual fast transient V4641 Sgr constrains its
mass to 8.7-11.7M_sun (9.6M_sun is the best fit value) and the distance to
7.4--12.3 kpc (Orosz et al. 2001). At this distance the peak flux of 12 Crab in
the 2--12 keV energy band, measured by ASM/RXTE, implies the X-ray luminosity
exceeding 2-3e39 erg/s, i.e. near or above the Eddington limit for a 9.6M_sun
black hole. An optical photometry shows that at the peak of the optical
outburst the visual magnitude increased by Delta m_V > 4.7^m relative to the
quiescent level and reached m_V < 8.8^m. An assumption that this optical
emission is due to irradiated surface of an accretion disk or a companion star
with the the black body shape of the spectrum would mean that the bolometric
luminosity of the system exceeds L>3e41 erg/s > 300 L_Edd.
We argue that the optical data strongly suggest presence of an extended
envelope surrounding the source which absorbs primary X-rays flux and reemits
it in optical and UV. The data also suggests that this envelope should be
optically thin in UV, EUV and soft X-rays. The observed properties of V4641 Sgr
at the peak of an optical flare are very similar to those of SS433. This
envelope is likely the result of near or super Eddington rate of mass accretion
onto the black hole and it vanishes during subsequent evolution of the source
when apparent luminosity drops well below the Eddington value. Thus this
transient source provides us direct proof of the dramatic change in the
character of an accretion flow at the mass accretion rate near or above the
critical Eddington value as predicted long time ago by the theoretical models.Comment: 4 pages, 2 figures. Submitted to A&A Letter
A decelerating jet observed by the EVN and VLBA in the X-ray transient XTE J1752-223
The recently discovered Galactic X-ray transient XTE J1752-223 entered its
first known outburst in 2010, emitting from the X-ray to the radio regimes. Its
general X-ray properties were consistent with those of a black hole candidate
in various spectral states, when ejection of jet components is expected. To
verify this, we carried out very long baseline interferometry (VLBI)
observations. The measurements were carried out with the European VLBI Network
(EVN) and the Very Long Baseline Array (VLBA) at four epochs in 2010 February.
The images at the first three epochs show a moving jet component that is
significantly decelerated by the last epoch, when a new jet component appears
that is likely to be associated with the receding jet side. The overall picture
is consistent with an initially mildly relativistic jet, interacting with the
interstellar medium or with swept-up material along the jet. The brightening of
the receding ejecta at the final epoch can be well explained by initial Doppler
deboosting of the emission in the decelerating jet.Comment: Accepted for publication in MNRAS Letters. 5 pages, 2 figure
Possible 38 day X-ray period of KS1731-260
We report the detection of a 38 day period in the X-ray flux of the transient
burster KS1731-260. The narrow peak of periodicity was detected during ~TJD
10150--11050 when the source had a high and relatively stable X-ray flux. After
\~TJD 11100 the source became strongly variable on a time scale of months that
contaminates the search for the 38 day periodicity. The detected period can not
be a binary period. The binary with Roche lobe overflow has in this case large
radii of the secondary and of the accretion disk. Disk and secondary star
illumination by X-ray flux from luminous neutron star would lead to high
infrared brightness of the binary. That clearly contradicts the infrared data
even for the brightest infrared sources within CHANDRA error box of KS1731-260.
Remaining possibility is that observed periodicity is connected with the
accretion disk precession, similar to that was observed for SS 433, Her X-1,
Cyg X-1 etc.Comment: 4 pages, 5 figures. Submitted to A&A Letter
Initial low/hard state, multiple jet ejections and X-ray/radio correlations during the outburst of XTE J1859+226
We have studied the 1999 soft X-ray transient outburst of XTE J1859+226 at
radio and X-ray wavelengths. The event was characterised by strong variability
in the disc, corona and jet - in particular, a number of radio flares
(ejections) took place and seemed well-correlated with hard X-ray events.
Apparently unusual for the `canonical soft' X-ray transient, there was an
initial period of low/hard state behaviour during the rise from quiescence but
prior to the peak of the main outburst - we show that not only could this
initial low/hard state be an ubiquitous feature of soft X-ray transient
outbursts but that it could also be extremely important in our study of
outburst mechanisms.Comment: 12 pages, Accepted for publication in MNRA
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